CA1170562A - Recirculating burner - Google Patents

Abstract

RECIRCULATING BURNER

Abstract of the Disclosure An internally recirculating burner designed to operate at a low level of excess air without producing intolerable levels of particulate (smoke) and oxides of nitrogen using ambient or highly preheated atmospheric air as an oxidant. The burner operates with a flame front outside of the burner in the com-bustion chamber with recirculation of furnace gas being pro-vided by the geometric configuration of the burner and the energy provided by the incoming combustion air. A nozzle means for controlling the combustion air flow is disclosed con-centric with the burner center body. The nozzle is comprised of a fixed ceramic nozzle plate having annularly arranged distribution holes and a nozzle plus also preferably of ceramic supported to open and close the annular flow passage between the nozzle and the burner center body as well as the annular flow distribution holes. The air discharged through the nozzle flows along the surface of the center body creating a pressure depression at the point of discharge causing the furnace gas to flow from the furnace chamber to the passage formed by the burner barrel and the outside diameter of the recirculating sleeve. The recirculated gas joins the incoming combustion air and flows parallel to the combustion air through the annular passage formed by the inside diameter of the recirculating sleeve and the outside diameter of the center body. Some mixing of the combustion air and the recirculating gas occurs within the recirculating sleeve. A
flame holder forming a portion of the center body central tube at the discharge end of the burner creates eddies and provides for a flame holding zone in which the fuel can be injected and the flame sustained. The flame thus formed extends into the combustion zone to the point at which the combustion reaction is completed.

Description

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-- i Background of the Invention - The present invention relates to a recirculating burner design preferably for use in an energy conserving process fur-nace of the type that may employ a recuperator for preheating the combustion air to a temperature of 600-2,400F in a steel forging furnace.
In an energy conserving process furnace employing a recuperator, and a recirculating burner as shown in U.S. Patent - No. 4,060,379, issued November 29, 1977, and assigned to the .`
same assignee as the instant invention, there is described a recirculating burner which operates satisfactorily in the combination. However, there is described herein in accordance with this invention an improved form of a recirculating burner such as might be employed in the furnace system described in this patent. The improvements in the recirculating burner are exemplified by the following and other objects which will become more apparent upon a reading of the details thereof described hereinafter.
One object of the present invention is to provide a recirculating burner that is substantially reduced in size and that, in particular, has been shortened in length. This -retuction in the size of the burner has been accomplished without any degradation in the operation or efficiency of the burner. The reduction in size now simplifies servicing of the burner and permits extracting of the burner from the furnace without requiring as substantial a clear space for purposes of burner remova-l as hereintofore necessary.
Another object of the present invention is to provide an improved means for controlling combustion air metering into the burner. In this connection, there is described an improved nozzle arrangement preferably employing a plug valve which provides improved linear adjustment for air intake into ~;
the burner. The improvement provides for better distribution throughout the annular flow passage entering the burner and permits a more linear relationship between the physical ,~ ,~;
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- position of the valve and the quantity of air passing the valve~ as well as a lower level of leakage when the valve is in the closed position. In this connection it is noted that the U.S. Patent No. 4,060,379 provides a nozzle for directing 5 combustion air but does not disclose a means for control associated with the nozzle.
A further object of the present invention is to provide a - recirculating burner which is adapted to receive preheated ~-combustion air and which preferably has a flame front main- '~
- 10 tained outside of the burner in the furnace cavity to increase recirculation of furnace gases and to maximize efficiency of the burnerr Still another object of the present invention is to pro-vide an improved recirculating burner which lowers energy L
15 requirements necessary to maintain a desired temperature such ~;
as in the process furnace applications and other similar r applications.
- A still further purpose of the invention is to provide ample mixing within the furnace cavity to avoid isolated 20 stratified pockets of gas which cause non-uniform heating of 4 the thermal energy absorbing surfaces of the furnace. ';
Still a further object of the present invention is to provide a recirculating burner which is operable at a relatively high temperature of intake air to thus achieve considerable 25 fuel savings and optimize the combustion process.
Another object of the present invention is to provide an improved internally recirculating burner design which operates with a low level of smoke and which is able to operate with either ambient or highly preheated air even at temperatures in the range of 600F to 2,400F or possibly higher. Although it is preferred that highly preheated air be used, it has been found that there is improved furnace efficiency even when com- L~
bustion air and ambient temperature is employed. ~' A further object of this invention is to provide a burner capable of operating on or near the optimum fuel air ratio or .
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7~5~ ) stoichiometric ratio without generating excessive quantities - of nitrogen oxide as well as particulate(smoke).
Still a further object of the present invention is to p,rovide an internally recirculating ~urner which is capable of 5 burning a wide variety of fuels such as natural gas and a mixture of coal and residual oil in 50%-50~ portions.
Another object of the present invention is to provide an improved recirculating burner design that operates over a wide -, ran~e of fuel and air flow independently while maintaining - 10 flame at a single geometric setting of the nozzle and other :~ components, thus providing ample turndown of the energy released by the burner and ample variations in the ratio of fuel to air.
Still another object of the present invention is to pro- f 15 vide an improved internally recirculating burner design in association with the furance system and wherein the-b,urner is : easily interchangeable with other burner designs.
; A still further object of the invention is to combust a variety of fossil fuels within the furnace cavity so completely ; 20 as to eliminate the "gray haze" normally generated by contemporary burners which interfers with the transmission of , energy from the flamé to the heat absorbing surfaces of the ~
, furnace by radiation. ' A further object o the invention is to provide a burner 25 which approaches a "perfect mixer". In such a burner, -~ stoichiometric quantities of fuel and air are reacted to form the products of complete combustion without producing unburned hydrocarbons and releasing surplus oxygen (excess air) to carry out and waste the energy released by the combustion pro-cess. The subject of this invention approach is the perfect mixer concept, and it affords an opportunity to effectively use sorbents to absorb the sulfur which may be present in the, fuel. Sulfur removal by the use of additives which would , ~:-react in the flame zone to absorb the sulfur in the fuel has long been a goal of researchers. Laboratory demonstrations ~ ~ 7D562 -6- r-have shown the feasibility of emoloying sorbents directly in - the fuel; however, in practice, such methods yield absorption - rates of 75% to 80%, which is not adequate. Further, large quantities of the sorbent material have been required to accomplish these results. In most instances, 3-S times the stiochiometric quantity of sorbent is required to obtain a level of 80% sulfur absorption.
Demonstrations have been performed with the invention -,-- wherein sorbents were employed at concentrations of 1-1/2 to ~-:

2 times the stoichiometric quantity. These demonstrations were performed with a 50~-50% mixture of coal and number 6 oil.
The results of the test revealed that 96~ of the sulur in the fuel could be removed in the flame zone by the use of sorbents with the burner.
v. 15 Summary of the Invention ~-To accomplish the foregoing and other objects of this invention, there is provided an improved internally recirculat-ing burner which is preferably used with an energy conserving 20 process furnace, although the burner of this invention may be ;~
~r~used with any type of furnace system which generally has a well-defined combustion zone such as the combustion zone of a steam generator, or an aluminum melting furnace. The burner of this invention is typically used in an energy conserving process furnace that incorporates a recuperator which is used to recover heat energy from the escaping gas such as may be used in a glass melting furnace process.
The recirculating burner of this invention is capable of operating with air preheated to temperatures in the range of 600F to 2,400F and even higher. The burner may be used with-out recuperator and without preheated combustion air to advantage L_ The recirculating feature provides some preheating of ~.
the incoming combustion ahead of the flame zone, even when used ~~
without a recuperator, and some dilution due to diffusion and ., ~J7~6~
. r 7 mixing of the air and the recirculated gas stream in the burner. In this manner, the concentration of oxygen in the mixture of gas and air approaching the flame is optimum wit~h a ~.
high concentration of oxygen at the core. The oxygen concen-: S tration gradually diminishes with increasing radial distance ~ away from the core so that the concentration of oxygen at the - outer extremes of the flow pattern is a minimum. The com-bustion zone is therefore sheathed in a cylinder of high temperature recirculated furnace gas. The effect of providing ~-'' 10 a containment sheath of very high temperature recirculated gas '`
rich in C02 around the flame zone (core) which is deficient in oxygen is to provide a highly radiant clean-burning flame.
The C02 in the sheath at elevated temperatures is a very '' effective ozidizer of carbonaceous smoke-forming particles 15 formed in the core. The C02 disassociates to C0, releasing ,~
.' atomic oxygen, which participates in the combustion ~rocess.
' The fuel thus formed is oxidized to C02 n~ar the completion of the combustion process. The participation of the high temperature C02 in the combustion process permits clean burn-20 ing with low levels of excess air. r ~' In accordance with the present invention,'the burner is ' ' now constructed so that it has been shortened in length considerably from the configuration disclosed in U.S. Patent ' No. 4,060,379. In this way, the working distance between the ;' 25 furnace outer wall and the nearest obstruction has been reduced for removal of the burner when maintenance is required. ''' With the recirculating design of the invention, heat release and heat transfer rates can be readily maximized because, the dynamics of the combustion process eliminates the "gray haze"
30 and stratified pockets of gas in the furnace zone. The im-proved heat transfer rates reduces the time required to heat ' ' ' work in process such as forging steel which in turn reduces the amount of oxide scale formed on the steel thereby improv- .
ing the life of the forging dies and the material lost due to 35 the surface'oxidati'on.

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, `' l37~5 - Substantially all of the components of the burner - exposed to the heat of the furnace and the preheated com-bustion air are formed from heat resistant and thermal shock resistant materials, or are formed of metallic materials which are ceramic coated to protect the materials from the . high temperature of the gas and the preheated air.
The burner is comprised of six principal sub-assemblies - which are:
1. A metallic internally insulated frame;
: 10 2. A combustion air nozzle consisting of a nozzle plate and plug;

3. A burner barrel;

4. A recirculating sleeve with locating means;

5. A central tube;

6. A core assembly comprised of the fuel delivery ~
system, the ignition flame safeguard and fine air ratio r sensors.
; The burner is designed to be inserted through the wall of .the furace cavity with provisions in the design to adapt to a variety of furnace wall thicknesses. When in position in a . furnace wall, the burner boundaries are composed of the ~; furnace cavity, the furnace walls, and the room or external `
atmosphere in which the furnace and burner assembly are located.
The burner is provided with certain services which vary with the application, local regulations and the type of fuel ~ which is to be burned. Typically, these services are com-f prised of; a fuel supply, delivered under pressure to the burner in a liquid, gaseous or fluidized powdered state, . 30 combustion air, cooling air, electric power for ignition, a pilot fuel which is normally a gaseous fuel, an electric sensing circuit to detect the presence of or absence of a flame, and an electric sensing circuit to measure the concen-', tration of oxygen in the furnace cavity immed.iately adjacent to the burner.

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- 1 ~7~562 - g A cylindrical central tube is coaxially mounted in the burner housing and is bolted in place. The core assembly is coaxially mounted in the central tube and is typically held in place by two latches for ease of assembly and disassembly. The central core incorporates all of the variations necessary to change from one fuel to another. Changing from natural gas to fuel oil,for example, involves changing the core only to accommodate the fuel to be burned. The fuel nozzle which intro- ;-duces fuel into the mixing zone is located at the center of the flame holder and protrudes into the furnace cavity slightly so as to deliver the fuel at the precise location required to sustain a stable flame.
A cylindrical burner barrel extends from the housing throùgh the furnace wall. The barrel is held in place in the 15 housing by metallic latches. The barrel provides a mounting ~-surface for the combustion air nozzle plate and the ~~
recirculating sleeve. The recirculating sleeve is coaxially located within the barrel at the furnace end of the burner and the nozzle plate at the opposite end of the burner. Annular passages are formed between the barrel and the recirculating sleeve and between the recirculating sleeve and the central L~
tube. Combustion air J under pressure, enters the burner at the combustion air inlet flange of the housing and flows through the housing to the combustion air nozzle. A plug valve positioned by and actually movable on the central tube is positioned to permit air to flow through the nozzle plate and along the surface of the central tube. By virtue of the "Coanda effect" the combustion air has a tendency of follow-ing the surface of the central tube until the flow encounters the spoiler at the end of the central tuve. At this point the flow is detached from the central tube and vortices are formed which provide the necessary flame holding conditions at - which point fuel can be injected. The pressure of the com- -bustion air as delivered to the burner is largely concerted to kinetic energy of the flow stream as the air passes through t ~ ~ ~ r~ ~j 2 ~- - 1 0 -the nozzle plate. As a consequence, by well established rules of physical science, the pressure in the immediate vicinity of the jet ensuing from the nozzle plate is depressed below the pressure existing in the furnace cavity. Pressure depressions of one-tenth to five-tenths of an inch of water and higher have been measured at points between the recirculating sleeve and the nozzle plate. The pressure depression causes furnace gases consisting of the products of combustion to fIow through the annular passage between the burner bow and the outside ~-diameter of the recirculating sleeve to join the incoming jet of combustion air. The furnace gases are entrained by the air and flow parallel to the air forming a sheath of high temperature gas surrounding and flowing coaxially with the air.
Mixing occurs between the recirculated furnace gas and the , 15, combustion air to a predetermined extent. Due to the differences in temperature of the two gas streams, mixing is '-minimal so that the flow nearest the central tube is largely combustion air with values on the order of 1~% to 20% oxygen by volume being measured near the surface of the central tube;
whereas, the composition of the flow nearest the recirculating sleeve internal surface of the dissharge from the burner is "~ , ,~ composed essentially of recirculated gas having the same ; composition as th'e gas found in the furnace cavity.
,' The combustion air with the sheath of recirculated gas , ;25 ,enters the flame zone with the weight flow of the gas being roughly equal to the weight flow of the air so that each flow of incoming combustion air is accompanied by one pound of fur-,, nace gas which has been recirculated from the furnace cavity;
however, ,the proportions have been varied from ratios of 0.2 to 2 of recirculated gas to combustion air flow. As the combined flow stream exists the burners, the flame holder causes the air stream to separate forming eddies at the point where the fuel is introduced. The disruption of the flow stream pro- .;,' vides a flame holding zone to initiate and sustain the com- r_ 35, bustion process.

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, ~ 7~5~2 ~) ' ,- - 1 1 -To obtain clean combustion producing the lowest levels of - particulate or smoke, and the lowest levels of nitrogen oxides the flow is not disrupted to such an extent as to destroy the sheath of recirculated furnace gas surrounding the flame zone.
5 By perpe-tuating this sheath into the furnace cavity, a highly luminous flame can be produced. The central core being deficient in oxygen, burns rich forming fine carbonaceous particulate which are effecitve thermal radiation sources.
This permits the efficient transfer of the energy released by r 10 the flame to the furnace walls by radiation. The high tempera-ture sheath of recirculated gas contains C02 which dis-associates providing atomic oxygen which effectively oxidizes particulate to C0 and C02-. As the flame progresses downstream ; into the furnace cavity, the C0 is oxidized to C02 by the 15 remaining oxygen. ~.
The combustion staging effect of the process described c~
produces the lowest levels of nitrogen ox de. By avoiding the presence of an excess of oxygen in the high temperature core, the fixation of atmospheric nitrogen is suppressed. So 20 effective is the burner in this regard that mixtures of coal and oil have successfully been burned with excess air levels of less than 2.5~ with essentially no carbon carryover to the exit gas and nitrogen oxide levels of less than one-half of "
that produced by comparable conventional burners in identical 25 service.
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Brief Description of the Drawings Numerous other objects, features and advantages of the invention should now become apparent upon a reading of the fol-30 lowing detailed description taken in conjunction with accompany-ing drawings, in which:
FIG. 1 is a cross-sectional view taken through a burner construction in accordance with the present invention; and ?~
FIG. 2 is a graFh that is associated with the operation 35 of the burner.

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Detailed Description With reference to the drawings, there is shown an internally recirculating burner, the components of which are predominantly made of ceramic or a like high-temperature --5 resistant material. This burner may be of the general type used with an energy conserving process furnace such as the one shown in U.S. Patent No. 4,-060,379. The burner may be supported from the furnace in a substantially conventional -~
manner such as- one similar to the support shown in the 10 reference U.S. patent.
The burner comprises a center body or center tube 10 supported in a substantially fixed position relative to a center tube flange mounted on the burner frame assembly 69.
The'forward end of the center body-10 defines a flame holder ~;
15 or spoiler 14 of annular construction. The center body 10 ; carries means including a core assembly 16 to provide fuel r-flow ignition means and flame monitoring of the combustion zone 18. The core assembly is held in place by two latches 71 which couple the core to the central tube flange 12.
20 Along the section of the center body 10 nearest to the furnace, there is provided a recirculating sleeve 20 which 7 encircles the center body. It is noted that the recirculating sleeve 20 terminates short of the flame holder 14 and at its ~.
oppPsite end terminates short of contacting the nozzle plate 25 34. Encircling the recirculating sleeve 20 is the burner barrel 24 affixidly supported at its inner end 25 by clips welded to the burner frame 27. In the drawing there is also shown an inlet passage 28 which directs combustion air into the plenum area 29 from which the air flows through the nozzle 30 entering through the ports 38 of the nozzle plate 34. The plenum area 29 is generally of annular configuration.
The nozzle means includes an annular nozzle plug 32 which is adapted to be adjusted for a particular air flow, and an ~, annular nozzle plate 34 including at one side a distribution ring 36 provided with a plurality of spaced distribution holes ,, 38. The nozzle plug 32 is preferably positioned by two - positioning rods 40 which are securely fixed in the nozzle p plug, one each at diametrically opposed locations. The outer ends of the rods 40 extending outside of the burner frame, may 5 be tied in common to an adjustment bar which can be positioned manually by the burner man or by other means such as an electric positioning motor activated from a remote location.
As the air leaves the plenum area 29 it passes through the r~'~
distribution ring holes 38. These holes have their open area ~-10 regulated by the adjustment of the plug nozzle 3~ relative to ' these distribution holes, with this adjustment also providing relative movement between the plug nozzle and the nozzle plate 34, and in particular at the tip 32A and 34A of the respective - plug nozzle 32 and nozzle plate 34. The flow exists thorugh the 15 nozzle means 30 and follows the center body 10 with the t-flow essentially terminating at the flame holder end of the r-center body. As the air flows on the surface of the center tube 10 some mixing occurs between the air and the neutralized , gas. The amount of mixing is an important consideration in the 20 design and operating characteristics of the burner. The air ~
premixed with recirculated gas flows over the flame holder 14 ~:
ant mixes with the fuel 15 and hot reacting gases behind the , , flame holder to form a stable flame that projects into the combustion zone 18.
~he fuel that is used by the burner flows through a fuel tube 44 disposed within the center body core 16 which is disposed concentric within the center body 10.' The fuel is coupled to a fuel source which is not shown,in the drawing.
At the flame hold,er end of the center body the fuel is injuected into the zone 18. Atomizing air may be optionally used with liquid fuels and mixtures of liquid and solid (particulate),fuel to cause or assist atomization. Various ~.
means of fuel preparation may be employed, all of which would ',.
be primarily contained within the core assembly 16. r-Thus, there is provided between the center tube 10 and 1 ~ 7~562 the recirculating sleeve 20 an eductor channel 46 through which the air and recirculated gas flow passes. Also, there is the recirculation channel 48 disposed between the recirculat-ing sleeve 20 and the burner barrel 24. The recirculated gases are aspirated into the recirculation zone 46 by the combustion air. An exchange of momentum between the incoming combustion air and the furnace gas provides the motive power to recirculate ~ the furnace gas. The-combustion products from the combustion ; zone 18 enter the burner at the recirculation zone entrance 49 and flow between the barrel 24 and the recirculation sleeve 20 through the zone 48 toward the nozzle plate 34. At the end 21 of the sleeve 20, the recirculated furnace gas turns radially inward into the recirculating sleeve 20. Note tlle arrows ~ indicating this action in FIG. 1. .
"; 15 Within the ceramic center tube body 10 in addition to the fuel lines and atomizing air lines, there is provided a means for ignition 50. There may also be provided a sight port 52 for viewing the flame. Actually, in the embodiment described the ceramic center tube 10 is supported by a metallic tube extension and flange bolting to the burner frame to support the entire assembly concentrically in the burner assembly. 2 The ignition means may be of any several different types incluting two spark electrodes tnote the spark electrode 54).
; Alternatively, a single spark electrode may be provided, a gas 25 line with a flame stabilizer at the end along with a sparking .`
means may be provided, or a clear passage for a manually-, operated torch may be provided. The center tube is preferably cooled by a relatively small amount of unheated air flowing within the tube around the core components. For coupling the cooling air there are provided inlets 56 to the center body cap 58.
The drawing also shows a burner face plate 60 which covers the burner plenum 29. The face plate 60 also provides ;
support means for the center tube and core. The burner plenum ~~
and face plate are insulated to prevent heat loss from the /~ ' '".
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preheated combustion air and to prevent the burner flame from exceeding safe temperatures for operating personnel. The drawing shows insulation 62 also for insulating the plenum area.
The flame holder 14 is in the form of an annular flange at the furnace end of the center tube 10 extending radially a height "~", which may be approximately 1/2 inch from the outer surface of the center body. It is disposed in this manner to cause flow ~ortecies for mixing the air, the gas and the fuel, and to hold the flame in position on the burner.
The plane of the flame holder is thus typically disposed slightly downstream of the end 23 of the recirculating sleeve 20. The downstream end of the recirculating sleeve 23 is typically disposed axially upstream from the flame holder, a r.
15 distance equal to or greater than the radial height of the ;-flame holder as measured from the outside diameter of the tube to the top of the flame holder lip, i.e., height "X". This arrangement prevents the streamlines of the flow through the recirculation zone from being moved radially by the flame Z0 holder toward the recirculating sleeve before they pass the exit plane. Premature radial movement of these streamlines t interferes with the control of mixing. ,~
The fuel tip 17 is disposed downstream slightly from the flame holder 14, preferably at a distance sufficient to pre- .
25 vent impingement of liquid spray in the case of a liquid fuel on the inner diameter of the center tube 10. The fuel tip p~referably has a spray angle sufficient to inject the spray into the shear layer defined by the eddy region behind the flame holder at a distance downstream of the plane of the 30 flame holder no more than from one-to-four radial heights of the flame holder, i.e., "X" to "4 times X". When gas is used in place of a liquid fuel the same relationship can apply ,.
except that a small amount of interference between the fuel jets and the inside diameter of the center body is 35 tolerable.

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The spray angle from tip 17 may be at a 120 included ~ angle and the tip may extend 1/8" beyond the end plane of the ,~
flame holder. Also, when using a gaseous fuel nozzle there k will be provided multiple radially projecting jets ~for 5 example, one row of eight holes) emanating from a position be-yond the plane of the flame holder by a mean distance of 1/8"
to 1/4" with no overlap of the jets with the plane of the flame older.
, Another feature of the present invention is concerned lO with a premixing scheme for premixing the combustion air and recirculated gases with the fuel. In this connection it is noted that the center body 10 is provided with a plurality of radially directed holes 64. Rather than being radially directed these holes could also be inclined. The holes are .-"
15 located ahead of the flame holder by an average distance of one-to-four times the radial height of the flame holder. These holes permit air and recirculated gas from outside of the center body to flow to the interior of the center body. This provides some premixing of combustion supporting gases and 20 fuel beore the fuel actually enters the extreme edges of the flame holding zone where full exposure of fuel to the com-bustion supporting gases takes place. This arrangement has the tual effect of helping to stabilize the flame and at the same time shape the flame. It has been found that the flame 25 shape with the radial holes 64 is "bushy" in comparison to a more "jet,-like" pattern when the holes are not used. There may be provided six 1/2" holes oriented radially at a centerline distance of 1-3/4" from the front edge of the flame holder. Also, one may provide in place of the six 1/2" holes, 30 say twelve 1/8" to 1/4" axial holes through the flame holder flange oriented axially around the flame holder flange of the center tube. ., In accordance with another feature of the present invention, by proper positioning of components of the burner 35 the recirculation and mixing can be controlled. The position ~2 of the entrance and exist of the recirculation sleeve with respect to the annular nozzle controls the recirculation rate and the mixing rate of the combustion air (motive fluid) and the products of combustion ~secondary fluid). It is preferred to recirculate approximately equal parts of combustion pro-ducts per unit of combustion air. This is a corollary to the desire to have a ratio of combustion product to combustion air between 0.5 and 1.0 at the lip of the flame holder. In one r-design there is a one-to-one mix at the lip of the 1ame holder because this represents a condition approaching that required for maximum reaction rate in the flame. Actual mixtures for maximum reaction rates are three or five-to-one (combustion products to combustion air) for an adiabatic flame.
For flame tempera*ures considerably lower than adiabatic ~as in the case of boilers and furnaces where heat is extracted directly from the flame) and with the further condition that r-there is a one-to-one mixture in the flame holding zone, i`t is necessary to add additional fuel and thus higher temperatures are achieved to reach actual maximum reaction rates and 20 highest flame stability. This occurs for any flame holder in ~;
the shear layer separating the flow of combustion support gases and recirculating hot product gases behind the flame holder. The heat start given the combustion supporting gases '`
approaching the flame holder by mixing in hot combustion 25 product gases enhances flame stability. ~`
It has been shown experimentally that too high a proportion of combustion products in the approach gas can actually destabilize a flame, as would be expected for too little oxygen being present. Thus, there is an optimum mixturé and the ratio of one-to-one is approximately correct for boilers and furnaces having chamber temperatures in the 2,000 to 2,600F range. .
The admixture of combustion product gases into the `.~
combustion air has another important effect. That is, smoke r-generation can be reduced to low levels by this method. At ~37~5 gas temperatures over 1,800F, as will occur in any fully developed flame, carbon dioxide is as effective as air in gasifying solid carbon. In any visible yellow or white flame there are carbon particles due to incomplete combustion or ~;
thermal cracking of fuel. This particularly occurs in oxygen depleted zones. In thè burner described herein, the core of the flame is depleted of oxygen because the fuel is admitted at the centerline, and practically all the air is admitted in - an envelope surrounding the core; in this case represented by the shear layer formed at the flame holder. Within this shear layer fuel can be thermally-cracked readily to form carbon particulate. If a flame originated under these conditions were to be cooled more rapidly than the carbon could be gasified, there would be particulate (smoke) passing through lS the flame envelope. This particulate would be discharged to the atmosphere as smoke.
Normally oxygen will react with carbon to form C0 or C02. At high temperatures C02 will react with carbon to form C0. In either case, the product C0 can burn out later in the flame without the generation of smoke. The amount of "gasifier"
available per unit volume ~ill tend to dictate how well the carbon will be gasified.
The quantity of gasifier available is represented by both air and C02. Assuming C02 is derived by complete combustion and then recirculated into the flame holding zone, the volu~etric quantity of gasifier available per unit of fuel supplied is 3n + 1 + ny' where the first term, n, is the volume of oxygen and the second term, ny, is the volume of C02. The term y is the volumetric proportion of exhaust pro-duct recirculated by a unit volume of combustion air. Thus, for a single volume of fuel (CnH2n+ 2 is assumed here) the ; ratio of total gasifier to that available without recircula-tion is: ,~
3n 2+ 1 + ny 3n ~ 1 5 ~ 2 Twice that available for air alone for y21. This has the ; a~vantage, with reactions at temperatures higher than 1800F.
of being twice as effective as air alone for eliminating carbon particles in the flame. Thus a burner can be operated with virtually zero excess air and-still be very effective as a clean burning system. The design of the burner of this invention allows the choice of the CO2 to O2 ratio, available in the primary combustion zone, defined by the shear layer behind the ~r-flame holder. :~
For control of CO2 to 2 ratio at the flame holder lip, a ~ey parameter is the mixedness of the recirculated product to the raw combustion air. The mixedness is controllable by the laws of fluid dynamics of mixing. For the instant burner the fluid dynamics is basically set by the dimensions of the 15 annular nozzle 30 surrounding the center body 10 with respect ,~
to the exit end of the recirculation sleeve 20. The half-strength ray of mixedness for annular jets eminates from the lip of the annular nozzle and spreads outward at an angle of 5.¢. This angle was measured experimentally by determining both the temperature and 2 concentration resulting from the mixing of a free annular jet of burning gas. The half-strength ray is defined as the locus of points measured radially from the center line of the jet having the strength of any character-istic compared to its center-line strength. The angle will depend somewhat on the actual gas composition, but the measure- ;
ment made for this device is satisfactory for the purpose at hand where the components being mixed are combustion products and air. The complete definition of the component di~tribu-tion radially through the spreading zone of the annular jet can be established analytically by recognizing that it is basically a Gaussian curve similar to that resulting from the spread of any gaseous jet. The interaction of the spreading jet with intersecting walls, in this case the inner side of the recirculation sleeve, is described basically as a reflection of the undisturbed composition curve shape from the wall back ~ 5~
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toward the center body. The absolute values of concentration are determined by adding the primary value and the reflected value of the Gaussian curve. l~hat this means is that the half-strength ray reflected off the wall totals to full strength at r 5 the wall by the addition of the primary and secondary values.
The actual value of the ratio of air to combustion products at the wall will, therefore, equal the actual value at the center body wall, assuming there is not a secondary reflection of the ,~
primary composition curve off the center body after its initial ~-10 reflection off the recirculation sleeve.
The center body wall value is determined easily by knowing the rate of decay of the initial concentration of gases from the annular jet. This again is similar to ~hat can be found in the literature concerning jet spreading. The position of the lS wall at the exit of the recirculation sleeve with respect to the half-strength ray can be selected to provide either majority combustion gas concentrations at the wall or perfect mixing, which would be 50 percent, simply by adjusting the radial posi-tion of the half-strength ray measured radially inward from the 20 inner lip of the recirculation sleeve. There is no theoretical value, so far as concentration is concerned, and based on free jet mixing measurements, in using a recirculation sleeve longer L~
than can be accommodated by the half-strength line intersecting "
the sleeve exactly at the lip since that represents perfect 25 mixing at the wall. On the other hand, the recirculation sleeve represents a confinement of a jet, so the actual mixing rate will be slower than for a free jet as represented by the above description. This would dictate slightly.longer sleeve require-ments than predicted by using free jet mixing measurements.
30 Any additional length will complete to a minor degree achieve-ment of uniformity of the mixture across the plane of the exit.
Added length is required to achieve complete velocity uniformity as in a conventional eductor, since velocity mixes slower than r~
mass or heat.

5 ~ ~ ` s Usually the recirculation sleeve is tapered outward toward the exit plane to help compensate for the mean change in gas .
density variation from the forward end to rear end of the recirculation sleeve. However, the forward end of the recircula-5 tion sleeve must have at least enough area to accommodate that required for the primary combustion alr jet and the parallel concéntric movement of combustion product being recirdulat-ed.
- Typically, the recirculated product amounts to the same mass flow as the combustion air. So the area required for the recir-10 culated combustion product at the entrance to the recirculation - sleeve corresponds to a mass flow equal to the combustion air mass flow at a temperature equal to the combustion chamber tem-perature and a velocity corresponding to the pressure drop available due to the aspirating action of the primary jet.
15 Typically, the recirculation zone at the forward lip of the C
recirculation sleeve has a pressure differential of approximate-ly ~.2 inch of water for a small burner. This corresponds to a radially inward velocity of 50 to 60 feet per second.
An equal area, or greater is provided for radially inward 20 flow at the forward lip of the recirculation sleeve through c the gap between the recirculating sleeve and the nozzie plate.
This flow takes place through an area defined by the imaginary cylindrical surface having the diameter of the forward lip of the recirculation sleeve and extending between the lip and 25 the nozzle plate 34, forming the rear of the recirculation zone.
Similar or greater area must also be provided in an annular re-gion between the recirculation sleeve and the inside of the bur-nér barrel.
The nozzle means 30 is one of the important features of 30 the present invention and is in the form of an air register mechanism for controlling the amount of flow through the annular jet. This arrangement comprises a plug nozzle 32 t' fitting through the annular jet orifice 31 and having a rear ~.
portion thereof that moves across the plurality of holes or 35 orifices 38 in the distribution ring 36. The diameter of the 3 ~ 7a5~2 - -round holes in the distributor ring and the stroke of the plug nozzle are matched so that the holes are fully closed when ~:
the plug exactly fills the annular jet and are-fully opened when the plug first offers zero obstruction in the annular jet as it is withdrawn. At intermediate positions of the plug noz-zle 32, the open area of the annular jet varies linearly with the axial position of the plug nozzle, and the open area through the distribution holes 38 equals the area of a segment of a P.
circle where the forward edge of the cylindrical portion of the plug nozzle forms a chord across the distributor holes 38. In the closed position the corical section 32A of the plug fits into the ar.nular jet with a line-to-line fit. The cylindrical portion of the plug fits radially within 0.005" at the forward edge of the distributor holss. When the plug nozzle 32 is moved toward the open position, that is, as it is withdrawn, the radial clearance with the distributor ring 36 can be designed to be as large as 0.050 inches, which makes easier the fabrica-tion of these parts from ceramics. FIG. 2 shows the air flow provided with this arrangement. For a fixed delivery pressure o the air supply, it should be noted that from approximately 10 percent open to 90 percent open, the flow varies linearly essentially with the position of the plug nozzle 32. This fea- `
ture is a considerable convenience for the control of air flow by a simple mechanism.
The plug nozzle 32 is moved by operating a yoke 41 connect-ed to operating rods 40 which in turn are connected internally to the plug nozzle 32 and locked by pins 68. The yoke 41 may be operated in any number of convenient ways including use of linear actuators, rotary actuators, or manually.
A radial seal ring of a suitahle material such as asbestos rope is disposed between the plug 32 and the center tube 10 to minimize leakage of air from the plenum 29 under the plug 32 to thus add to the air normally introduced by flow between the r plug 32 and the nozzle plate 34. Air leaking in this manner would reduce the operable range and the burner optimum fuel I ~ 7 D ~ ~ 2 '`

air ratio.
The entrance to the combustion air plenum 29 from the hot air supply duct 28 should have an area approximately three times the area of the distributor holes 38 in the full open position.
Fuel 15 is sent throug]l the tube 44 to the fuel injector at the furnace end of the center tube. Liquid fuels may be placed under pressure as high as 5000 p.s.i. or any other value which matches the fuel injector characteristics. The value -~
would depend upon the service under which the burner is to be put. Conventional burners using #2 or #6 fuel oil typically use air or steam atomization to prepare the fuel for good burning by forming droplets in the order of 20 to 60 microns. The air or steam may be fed at any pressure from 20 to 100 p.s.i. .-The fuel is fed in a concentric or parallel line down the center body leading to the injector at pressures also from 20 to 100 p.s.i. The fuel nozzle tip 17 thus can be any of a variety of commercial nozzles, such as the Delavan "Swirl Air" nozzle.
Pressure atomizing nozzles may also be used for #2 fuel oil. In this case, oil is fed at a low pressure of about 20 p.s.i. at minimum flow to as high as 400 to 500 p.s.i. at i~:
maximum flow. No secondary atomizing fluid need be used when pressure atomizing is employed.
High préssure atomizing systems such as the HI Super Critical Fuel System, U.S. Patent No. 3,876,363 granted April 8, 1975, LaHaye, et al, may be used with this burner. ~~
Typically, the fuel and the atomizing medium, if used, is supplied to the fuel injector through the core in concentric tubular passages, usually with the atomizing medium flowing outside of the fuel tube. These may extend through the center body ace plate of the burner at which connection can be made to acilitate the maintenance services, or to a manifold which also serves for other fluids required by the burner and which can be connected to external services in any of a number o ways.
Gaseous fuels will be fed in much the same manner except that a separate atomizing fluid would not be required.

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7~5 The center body 10 may also be equipped with safety devices and ignition devices extending from the center body face plate inward toward the burner tip. Typically, this includes spark electrodes 54 for igniting the fuel directly or for igniting 5 the pilot fuel, when used. The pilot fuel will be fed in a separate line running parallel to the center body core 44 along with the spark electrodes and atomizing air and fuel lines.
The gas pilot would terminate within an inch or two of the cen- :
ter body tip and would be ignited by the electric spark when 10 desired. The gas pilot provides a flame or torch which will in turn ignite the main fuel spray or jet. The gas pilot would typically be used with the heavier fuel such as #6 fuel oil and mixtures such as coal and oil slurries to assure ignition.
Room for sighting down the center tube may also be provided ', 15 for scanning the flame with a flame sensing device mounted on : the center body face plate. The scanner, ~hich is commercially available, is directed optically toward the center body tip.
The appearance of the flame will cause a current to be generated ~ in the flame scannér which can be detected by a suitable elec- L
i 20 tronic circuit and used as a flame monitor for safety purposes '~
in accordance with industry standards. Provision is also made for observing the appearance of the flame through a sight glass opening 52 so as to allow viewing the center body tip. The sight glass will also be mounted in the center body face plate.
Since this burner is preferably designed for use of highly preheated combustion air and a portion of the burner is exposed to combustion products directly connecting with the combustion chamber, the outer housing would be very hot were it not thermally protected by insulation. Insulation 62 is used in the combustion air chamber to keep the outer skin of the burner to temperatures of 300F. or below. Typically, this can be accomplished with l to 1.5 inches of alumina-silica fiber material fastened to the inner surface. '~
The mounting flange 27 of the burner is approximately : 35 in the plane of the nozzle plate 34 which is also the separating -~ 3 ~ 7~562 plane between combustion air and recirculated combustion gases.
Thus the combustion gases are exposed only to the walls of the furnace through which the burner barrel 24 extends and onto which the burner flange is fashioned. Thermal insulation 70 separates the burner frame 69 from the barrel 24.
Several burners have been tested with various fuels:
natural gas, propane, #2 fuel oil, #6 fuel oil, and with coal + #6 fuel oil mixtures. The #2 and #6 fuel oil runs have been documented in boiler operation. The #2 fuel oil and coal + #6 fuel oil slurries have also been documented in furnace operation.
Table 1 shows the results in terms of excess air, NOX, unburned - hydrocarbons, and smoke. The results`are compared against other burners. It can be noted that very low smoke levels can be achieved with very low excess air with this burner. NOX is not as high as would be expected for a given flame temperature, fuel flow, and chamber size. The operating range of this burner appears satisfactory over at least a 4 to 1 max. to min. flow range.

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The preferred embodiment of the burner depicted in the drawing has a number of features which are considered improve-ments over prior art constructions. The improved operation is provided by the particular arrangement of the fuel nozzle with respect to the position of the recirculation sleeve 20 and the air nozzle and associated annular air jet. The burner of this invention is characterized by efficient control of the mixing of combustion air and recirculated gases. A further r`, improvement in accordance with the invention is in the nozzle means or air register means which includes a matched construc-tion of distribution ring, annular nozzle, and plug nozzle for effecting controlled opening, at any axial position, to achieve flow characteristics which are linear with opening distance. ~-Having described one embodiment of the present invention 5.
it should now be apparent to those skilled in the art that ~~
numerous other embodiments are contemplated as falling within the scope of this invention. While the invention has been described for use in connection with a particular type of urnace, it is understood that the burner of this invention can be used in other applications.
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Claims (21)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A burner capable of operating with combustion air delivered to the burner at temperatures of from 600°F. to 2400°F. and higher, said burner comprising, a center tube defining a forwardly extending ceramic high temperature resistant tube portion having a flame holder at a forward end of said burner, an encircling ceramic high temperature resistant burner barrel extending about said ceramic tube, an encircling recirculating ceramic high temperature resistant sleeve extending about said ceramic tube between said tube and said barrel and defining a recirculating gas passageway such that furnace gases can be carried from a for-ward end of said burner between said barrel and sleeve to a rearward plenum area of said burner and then forwardly between said tube and sleeve completely surrounding said tube and then out of said forward end, said plenum area encircling said center tube being defined in part by a rear high temperature resistant ceramic wall, and combustion air means for providing a combustion air flow axially of said center tube between said center tube and said sleeve, said combustion air means comprising a ceramic high tem-perature resistant lined chamber and a ceramic high temperature resistant chamber plug, an inlet passage for introduction of hot combustion air to said chamber, and means for mounting said plug in a predetermined position to adjust the size of said inlet passage and thus adjust the passage of combustion air to said eductor channel.

2. A burner in accordance with claim 1 and further comprising said plug defining openings therein for distributing combustion air to said plenum substantially uniformly about said center tube.

3. A burner in accordance with claim 1 and further comprising means for reciprocating said plug so that a predetermined volume of combustion air flow to said plenum where-by said plug acts as a valve member.

4. A burner in accordance with claim 1 wherein said plug includes a nozzle plug movable relative to a nozzle plate associated with said chamber.

5. A burner in accordance with claim 4 wherein said noz-zle plug and nozzle plate are both annular with the nozzle plug having a conical end and a cylindrical end, said nozzle plate having one end cooperating with the conical end of the nozzle plug and another end defining a distribution ring cooperating with the cylindrical end of the nozzle plug.

6. A burner in accordance with claim 5 including yoke means for operating said nozzle plug.

7. A burner in accordance with claim 1 wherein said flameholder is defined by an annular flange extending outwardly of the center tube.

8. A burner in accordance with claim 7 including at least one mixing hole through the center tube spaced adjacent but rearwardly of the flameholder flange.

9. A burner in accordance with claim 1 wherein said center tube extends forwardly a greater distance than the recirculating sleeve.

10. A burner in accordance with claim 9 wherein the recirculating sleeve extends forwardly a distance equal to or greater than the burner body.

11. A burner in accordance with claim 1 wherein said combustion air means also includes a fixed nozzle plate at least partially forming the chamber, a mixing zone being pro-vided between an inner terminating end of the recirculating sleeve spaced from the nozzle plate, and the nozzle plate itself.

12. A burner comprising;
a center body including a forwardly extending high tempera-ture resistant tube having a forward end flameholder, a high temperature resistant burner body encircling the center body, means supporting the burner body and center body in concentric relationship, a high temperature resistant recirculating sleeve extend-ing about said center body between said center body and burner body and defining a recirculating gas passageway such that furnace gases can be carried from a forward end of said burner between said burner body and sleeve to a rearward plenum mixing zone of said burner and then forwardly between said center body and sleeve all the while mixing with combustion air, combustion air jet means including an inlet passage for introduction of combustion air, a high temperature resistant chamber that said inlet passage leads to and means associated with said chamber intermediate the inlet passage and plenum mixing zone for controlling combustion air flow to the mixing zone between the center body and recirculation sleeve.

13. A burner as set forth in claim 12 wherein said center body, burner body and sleeve are all of a high temperature resistant ceramic withstanding temperatures at least on the order of 2500°F. or less.

14. A burner as set forth in claim 12 wherein said flameholder is defined by an annular flange extending outwardly of the center tube.

15. A burner as set forth in claim 14 wherein said sleeve is tapered to a narrower diameter at its rearward end.

16. A burner as set forth in claim 14 wherein said sleeve has a forward end terminating short of said flameholder and a rearward end terminating short of the combustion air control means.

17. A burner as set forth in claim 16 wherein said burner body has a forward end terminating short of said sleeve and a rearward end forming a support for at least part of the combustion air jet means.

18. A burner as set forth in claim 12 including a plug and means for mounting the plug in an adjustable position to adjust the size of the passage through the combustion air jet control means to thereby adjust passage of combustion air to the area about the center body.

19. A burner as set forth in claim 18 wherein the nozzle plug has a conical forward end and cylindrical rear end cooperating with the holes in the distribution ring.

20. A method of operating a recirculating burner having a recirculating channel starting from a combustion zone and passing to a rearward plenum area and then forward to said combustion zone in an insulated passageway, said method comprising adding combustion air to said plenum area and having said air picked up by flow from said combustion zone to pass outwardly to said combustion zone, said combustion air being added in response to an opening of variable cross-sectional area provided by a movable ceramic ring, and moving said ceramic ring to expose a greater or lesser passageway for said combustion air to said flow through said recirculating burner.

21. In a recirculating burner having an inner body, a recirculating sleeve and an outer burner barrel to provide an inner recirculation passageway, an outer recirculating passage-way and a plenum connecting the two passageways at a zone distal from a combustion zone at one end of said burner, the improve-ment comprising a movable ceramic nozzle member facing said plenum and defining with a nozzle plug, a passageway there-through to said plenum whereby combustion air can be added to said plenum from a chamber behind said nozzle, and means for moving said nozzle plug with respect to said nozzle member in order to provide varying cross-sectional passageway to said plenum which passageway extends substantially about a circular path to provide uniform mixing of said combustion air with recirculated gases in said burner which mixing can be varied by said movement to determine the amount of combustion air added.